1. Field of the Invention
The present invention pertains to the polishing of thin-film magnetic heads, and in particular to polishing conducted in a batch mode.
2. Description of the Related Art
Digital data processing equipment, such as hard disk drives used with computer systems, is being produced in increasing numbers and a continuing effort is being made to improve device speeds while reducing manufacturing costs. An important component of such devices is the head stack assembly or slider assembly which includes one or more magnetic heads for storing and accessing data contained on a magnetic storage disk. Although magnetic heads can be manufactured singly, it has been found more efficient to produce such heads in a batch process.
In one type of production of magnetic heads, multiple magnetic heads are arranged side-by-side to form an array typically referred to as a "row". The magnetic heads, grouped together in a row form, are mounted in mass on a row tool or bar to facilitate lapping or polishing of the individual magnetic heads disposed in the row. After lapping or polishing, the individual magnetic heads are separated from the row.
Several types of magnetic heads have been developed over the years. In the recent past, thin-film inductive heads were the most popular type of magnetic heads in commercial use. However, the commercial usefulness of such heads began to wane as the market demanded magnetic storage devices of higher magnetic densities and higher device speeds. Both of these trends tended to diminish the data storage signals which could be developed in thin-film inductive heads and commercial market interest switched to thin-film heads of the magnetoresistive (MR) type. A significant technical advantage of MR magnetic heads is their ability to develop greater signal strength, independent of storage disk velocity.
As with previous thin-film devices, the MR magnetic heads are developed by depositing a number of layers on a substrate. The deposited layers form a magnetoresistive stripe located adjacent the pole tips of the head. In operation, the magnetoresistive stripe of material is made to conduct a steady sense current which sees a changing resistance in the stripe as the stripe passes through varying magnetic fields of the memory storage disk. Typically, MR heads include separate read and write elements and alignment of these elements, especially under ongoing pressures to miniaturize magnetic head devices, is of special importance. In particular, one device parameter, the so-called "stripe height" has been subjected to increasingly tightened dimensional tolerances. Essentially, the stripe height is the distance between the air bearing surface of the magnetic head which rides over the storage disk (i.e., the portion adjacent the pole tips) and the region where the magnetoresistive stripe is formed. The stripe height is defined by accurate lapping of the pole tips and adjacent end portions of the MR heads. Typical stripe heights are on the order of one micrometer and it is desired to attain a predetermined stripe height within an accuracy of one micro inch (approximately the 40th part of a micrometer). As will be appreciated, the lapping process must be carried out with considerable precision.
As mentioned above, for commercial production reasons, the pole tips defining the transducing gap of each MR head in a row formation are simultaneously lapped such that, when the lapping procedure is completed, all the heads of the row formation have the desired stripe height. Typically, the row tool on which the heads are mounted is manipulated so as to advantageously deform the row structure to apply differential lapping rates to different heads in the row formation. Such manipulation is required for a number of reasons. For example, the amount of material between the desired stripe height and the free edge of the row varies, due to manufacturing tolerances, and, oftentimes some of the heads in the row formation require greater amounts of material which must be removed in order to attain the desired stripe height.
Manipulation of the row tool is also required because the row tool is subjected to stress inducing processes which result in the tool assuming an undesirable curvature which must be corrected during the polishing operation. The need to advantageously manipulate the row tool has given rise to several mechanical solutions as can be observed in U.S. Pat. Nos. 5,203,119 and 5,117,589, for example.
One arrangement is disclosed in U.S. Pat. No. 5,607,340 which provides row tool having elongated relief slots advantageously positioned so as to provide an improved shape conformance ability. The row tool is manipulated by three bend rods which impart a mechanical force to the row tool, causing the bottom of the row tool to take on different shapes, as desired. Typically, the row tool is continuously manipulated throughout a polishing operation to alter the lapping rates of individual head members of the row of an ongoing, dynamic adjustment basis throughout the lapping procedure.
U.S. Pat. No. 5,620,356 provides correction apparatus for row tool balance and bow, using electromechanical deices to apply bending loads to the row tool. The electromechanical devices operate with automatic open-loop control, using signals obtained from the products being processed by the row tool. Open loop control system requires sophisticated calibration equipment that is difficult to use and maintain so as to avoid introduction of subtle errors. The electromechanical deices undergo a travel which is largely nonlinear with only a very narrow area of operation where the force output is substantially linear. Accordingly, the assembly and operation of the electromechanical devices must be carried out in a costly, time-consuming manner to ensure that the relatively small desirable area of operation is effectively employed. Despite these precautions, such systems are subject to downstream mechanical hysterisis which will adversely affect operating precision. Improvements to overcome these and other drawbacks encountered in the manufacture of sliders, slider assemblies, head stack assemblies, as well as the individual magnetic heads is continually being sought.